1. Field of the Invention
This invention relates to a projection exposure apparatus used for the manufacture of semiconductor integrated circuit elements, and particularly to an apparatus for detecting any variation in the optical performance of the projection optical system thereof.
2. Description of the Prior Art
Generally, the manufacturing process of integrated circuits such as IC and LSI is based on plural repetitions of resist application.fwdarw.alignment.fwdarw.exposure.fwdarw.development.fwdarw.etc hing effect for a wafer. In the collective exposure system wherein the pattern of a reticle is formed on a wafer in the relation of one to one, the error of alignment caused by deformation of the wafer occurring in the etching step or the like is great and coping with the minute patternization required of the exposure step is difficult. For this reason, the reduction projection exposure system has recently come to be used for the manufacture of highly integrated semiconductors. In the reduction projection exposure system, a plurality of exposed areas of the order of 10 mm-20 mm square are printed on a wafer of 4-6 inches by the step and repeat system and therefore, even for a deformed wafer, it is possible to maintain the alignment with the pattern of the preceding step at high accuracy during each exposure. Also, the projection lens for the reduction projection exposure system has N.A.=0.26-0.35 and uses g-line (.lambda.=436 nm) or i-line (.lambda.=365 nm) of short wavelength as the exposure light and therefore, even the so-called submicron printing of line width 1 .mu.m or less is possible.
However, to accurately effect the repetition of the exposure of a minute pattern in the submicron area, it is necessary that the optical performance of the projection optical system itself be always in a stable and excellent state. The factors which cause the fluctuation of the optical performance of the projection optical system include environmental conditions such as temperature, humidity and atmospheric pressure, and the optical performance is also fluctuated by the optical elements themselves of the projection optical system absorbing the exposure energy and varying their own temperatures, and it has been found that such fluctuation is not negligible for making the patterns for super LSI more minute. As the variations in said optical performance, there are the variation in the imaging plane and the variation in the projection magnification. The variation in the imaging plane has a fatal adverse effect in a shallow depth of focus which is essentially unavoidable for a high resolving power, and the variation in the magnification adversely affects the accuracy of superposition printing. The projection optical system absorbs the applied energy for exposure and accumulates it as heat energy and therefore, the variation characteristic of its optical performance fluctuates depending on the manner of application of the exposure light, i.e., the application history. Where the application history is constant, the amount of fluctuation of the optical performance corresponding to the exposed state can be known by empirically measuring the variation characteristic of the optical performance in advance and it is possible to provide correcting means necessary for the correction thereof. Generally, however, the time required for replacement or alignment of a wafer and a reticle is not always constant, and the application history may also be changed by an unforeseen situation and in such a case, it is no longer possible to know the varied state of the optical performance. Accordingly, even if there is some correcting means, it becomes impossible to know how and to what degree the correction must be effected and thus, it becomes impossible to maintain a desired optical performance.
Also, a technique of calculating the integrated value of exposure time from the application history and operating the amount of correction of the optical performance on the basis of this value is known from British Laid-open patent application No. 2,121,558, etc., but the fluctuation of the optical performance is caused not only by the exposure history but also by the variations in the transmission factor of the reticle and the brightness of the light source and therefore, detecting means is required for each fluctuation element and the construction becomes complicated and also, a measurement error is caused for each detecting means, and this has led to a disadvantage that it is difficult to accurately know the fluctuation of the optical performance.